The separation of acid gases such as sulfur dioxide, hydrogen sulfide or carbon dioxide from gas streams such as flue gas or hydrocarbon containing streams by means of absorption into aqueous amine solvents is well known. Many of these process, which are referred to as amine treater processes, are described in “Gas Purification”, 5th Edition, Ed. Arthur L. Kohl and Richard B. Nielsen, Gulf Publishing Company, Houston, Tex.
Amine treater processes use a regenerable amine solvent whereby the acid gas is captured into the solvent at one temperature and the acid gas is desorbed or stripped from the solvent, generally at a higher temperature.
The amine solvent for removing a given acid gas component from a feed stream may be chosen so that the acid gas can be removed from the solvent by steam stripping. If steam stripping is utilized, then in order to separate the acid gas from the solvent, the acid gas must be volatile while in solution. Preferably, the acid ionization constant of the conjugate acid of the amine (the pKa) has a value no more than about 3 or 4 units higher than the pKa of the acid gas. If this difference in pKa is larger than about 3 or 4 units, then the salt formed between the amine and the acid is too stable to be practically dissociated by steam stripping.
In commercial operation, acid gas capture processes experience ingress and/or in process generation of acids that are stronger than the acids for which the removal process is designed. These stronger acids form salts with the amine solvent which are not regenerable with steam and are thus termed heat stable amine salts, or just heat stable salts.
If the heat stable salts are allowed to accumulate, they will eventually neutralize all the amine of the solvent, rendering it unable to react with and remove the acid gas component as intended. Accordingly, provision for heat stable salt removal is necessary for systems where strong acids accumulate in the amine solvent.
Various means for removal of heat stable salts from amine gas treating solutions are known. These include distillation of the free amine away from the salt at either atmospheric or subatmospheric pressure (see for example “Gas Purification”, p. 255ff), electrodialysis (see for example U.S. Pat. No. 5,292,407) and ion exchange (see for example U.S. Pat. Nos. 4,122,149; 4,113,849; 4,970,344; 5,045,291; 5,292,407; 5,368,818; 5,788,864 and 6,245,128).
One problem with ion exchange processes is that the ion exchange medium or resin must be regenerated from time to time. During the loading stage of the ion exchange process, the anion removal capacity is used up as heat stable salts are removed from the amine solvent. Upon exhaustion or reduction of the anion removal capacity of the ion exchange resin by a particular amount, feed of the heat stable salt rich amine solvent to the ion exchange resin is terminated so that the ion exchange resin may be regenerated. Since the ion exchange resin regeneration agent will typically be sent to waste water treatment or otherwise be disposed of, the amine solvent in the amine resin bed may be displaced and washed by large volumes of water prior to commencing the flow of resin regeneration agent to the column, in order to recover a dilute amine solvent solution prior to the ion exchange resin being regenerated. If the dilute amine solvent solution is returned to the acid gas capture process, thereby avoiding loss of amine solvent, this introduces water to the amine solvent in the acid gas capture process thereby diluting the concentration of the amine solvent in the solvent solution in the amine treater process.
Amine treater processes are designed to work at a fixed, optimum amine solvent concentration. Therefore, dilution of the amine solvent is undesirable. Accordingly, water should be removed from the amine solvent if the diluted amine solution is returned to the acid gas treatment process or the dilute amine solvent stream may be discarded to waste, resulting in a loss of amine solvent.